Recovering Energy From Building Exhaust With Membrane-Based ERV
From ASHRAE Journal, December 22, 2020
Through membrane-based energy recovery ventilation (ERV), energy can be recovered from building exhaust via water vapor transport and heat transfer across gas permeation membranes, thereby reducing latent and sensible loads during both heating and cooling seasons.
A recent Science and Technology for the Built Environment article investigates the use of polydimethyl siloxane (PDMS) membranes packaged into hollow fiber modules. Jonathan Maisonnneuve, Ph.D., assistant professor in the Department of Mechanical Engineering Oakland University, discusses the research.
1. What is the significance of this research?
Significant energy can be recovered from building exhaust via water vapor transport and heat transfer across gas permeation membranes, thereby reducing latent and sensible loads during both heating and cooling seasons. The process known as membrane-based energy recovery ventilation (ERV) has been commercialized by several HVAC manufacturers, but so far has mostly been offered in flat sheet membrane configurations.
In this study, we investigate the use of polydimethyl siloxane (PDMS) membranes packaged into hollow fiber modules, which offer the advantage of high packing density, but which have so far not been commercially adopted due to concerns about parasitic pressure loss. In addition, we introduce here the concept of normalized net energy savings, which can be a useful metric for comparing performance independent of flow conditions and membrane size.
2. Explain the steps of this research project. What did the process look like?
The project involved psychrometric and thermodynamic analyses to evaluate the theoretical maximum energy recovery in the sweep driven process. Experiments were then conducted by supplying humidified nitrogen gas over a commercial polydimyethl siloxane (PDMS) membrane with hollow fiber configuration.
Four cases with different humidity gradients were chosen to observe the effect of building intake/exhaust conditions. Airflow rate was varied to determine its effect on vapor flux, parasitic pressure losses due to friction and overall net latent energy savings. Results were compared to the literature and energy savings were calculated for case studies in Detroit, Mich. and Houston, Texas.
3. Why is it important to explore this topic now?
About one-third of total end-use energy in the United States is consumed by buildings with HVAC accounting for up to 30% of this. Improving the energy efficiency of HVAC systems is therefore important for the cost-effectiveness, health and sustainability of buildings. Membrane-based energy recovery ventilation (ERV) can be used to recover both latent and sensible energy from building exhaust and thereby significantly improve HVAC energy efficiency.
4. What lessons, facts, and/or guidance can an engineer working in the field take away from this research?
By introducing the concept of normalized net energy savings, this study provides important context to the performance results reported throughout the literature. Typical metrics such as effectiveness and moisture removal are highly dependent on system scale, and these metrics can be inflated with an oversized (i.e. costly) system.
We suggest that normalizing performance per unit membrane area can provide a useful metric that can be explored to optimize design and operation of membrane-based ERVs.
5. How can this research further the industry's knowledge on this topic?
Most commercial membrane-based ERV units are currently offered in flat sheet plate and frame modules, with various flow configurations due to ease of construction and integration with HVAC systems. However, the required surface area of optimally sized membrane-based ERVs will likely require high packing densities, which can be greatly facilitated by hollow fiber modules.
This study contributes to the ERV literature by providing experimental demonstration of a widely available PDMS membrane material used in hollow fiber configuration for building ERV applications.
6. Were there any surprises or unforeseen challenges for you when preparing this research?
Experimental setup and investigation proved time consuming. Students designed and built the test bench from scratch. Once complete, experiments extended over a period of 4 months with each trial requiring from five to eight hours. Control of feed and sweep humidity was achieved by manually blending parallel streams of dry and humid air, but this was challenging because the valves on each line lacked resolution.